FIG. 19 illustrates a wheel hub unit that rotatably supports a wheel of a vehicle or the like on a suspension device.
The wheel hub unit includes a hub main body 1 and a tapered roller bearing unit BU incorporated in the hub main body 1.
In the hub main body 1, a spline 1b with which an end part of a drive shaft (not illustrated) is spline-engaged is formed on an inner peripheral surface of a hollow cylindrical part 1a, a flange 1c for supporting a wheel (not illustrated) is formed on the underside of the hollow cylindrical part 1a having a straight shape (a shape having the same outer diameter from the upper end to the lower end), a plurality of penetration screw holes 1d are formed in the flange 1c, and an external cylindrical part 1e is formed below the flange 1c. 
Inner rings 2, 3 are externally fit to the hollow cylindrical part 1a of the hub main body 1 with the inner rings 2, 3 spaced apart from each other in the axial direction, a distance piece 4 is externally fit to the hollow cylindrical part 1a at a position between the inner rings 2, 3, an outer ring 5 is arranged on the outside of the inner rings 2, 3, and a plurality of tapered rollers 6a, 6b are rollably arranged between the outer peripheral surfaces of the inner rings 2, 3 and the inner peripheral surface of the outer ring 5.
Regarding the inner ring 2, an inner ring raceway 2a on which the plurality of tapered rollers 6a roll is formed on the outer peripheral surface thereof, and a stepped part 2b with which the end surfaces of the tapered rollers 6a in the axial direction are in contact is formed on the outer end part of the inner ring 2 in the axial direction. In addition, regarding the inner ring 3, an inner ring raceway 3a on which the plurality of tapered rollers 6b roll is formed on the outer peripheral surface thereof, and a stepped part 3b with which the end surfaces of the tapered rollers 6b in the axial direction are in contact is formed on the outer end part of the inner ring 3 in the axial direction.
Regarding the outer ring 5, a supporting/fixing part 5c supported and fixed by a suspension device (not illustrated) is provided to protrude toward the outer peripheral side, and an outer ring raceway 5a on which the plurality of tapered rollers 6a roll, which is opposed to the inner ring raceway 2a, and an outer ring raceway 5b on which the plurality of tapered rollers 6b roll, which is opposed to the inner ring raceway 3a, are formed on the inner peripheral surface thereof.
Accordingly, the tapered roller bearing unit BU is configured by a first-row tapered roller bearing 8 including the inner ring raceway 2a of the inner ring 2, the outer ring raceway 5a of the outer ring 5, and the plurality of tapered rollers 6a, and a second-row tapered roller bearing 9 including the inner ring raceway 3a of the inner ring 3, the outer ring raceway 5b of the outer ring 5, and the plurality of tapered rollers 6a. 
When a swaged part 7 is formed by plastic-deforming the upper end part of the hollow cylindrical part 1a of the hub main body 1 outwardly in the radial direction, the inner rings 2, 3 of the tapered roller bearing unit BU, which are externally fit to the hollow cylindrical part 1a, are pressed toward the side of the flange 1c, and the first-row and second-row tapered roller bearings 8, 9 are assembled into the hollow cylindrical part 1a of the hub main body 1.
FIG. 20A illustrates a state where the tapered roller 6b of the second-row tapered roller bearing 9 is arranged with the end surface thereof in the axial direction not being in contact with the stepped part 3b, and FIG. 20B illustrates a state where the tapered roller 6b is arranged with the end surface thereof in the axial direction being in contact with the stepped part 3b (hereinafter, referred to as “arranged at a true position”).
If the tapered roller 6b of the second-row tapered roller bearing 9 is assembled to be arranged in a state where the end surface in the axial direction is not in contact with the stepped part 3b, as illustrated in FIG. 20A, preload applied to the second-row tapered roller bearing 9 is decreased and the preload is released when forming the swaged part 7, and movement in the axial direction and skew occur in the tapered roller 6b when forming the swaged part 7.
Therefore, in the tapered roller bearing unit BU incorporated in the conventional hub main body 1, a flaw may occur on the inner ring raceway 3a of the inner ring 3 and the outer ring raceway 5b of the outer ring 5. In addition, similarly, in the first-row tapered roller bearing 8, if the tapered roller 6a is arranged at the position illustrated in FIG. 20A, the above-described problem may occur.
In this regard, PTL 1 already filed by the present applicant states that, when an inner ring of a tapered roller bearing unit is externally fit to a cylindrical part of a hub main body (described as a step in PTL 1), an outer ring is rotated (paragraphs 0019 and 0020 in PTL 1).
As stated in PTL 1, aligning action is implemented on tapered rollers by rotating the outer ring when the inner ring is externally fit to the cylindrical part of the hub main body, the tapered roller bearing unit in which the tapered rollers are arranged at the true position as illustrated in FIG. 20B is incorporated in the hub main body, and therefore, a wheel hub unit that prevents release of preload, movement of the tapered rollers in the axial direction, and skew can be provided.